One-tick pulses into sticky pistons with slime blocks create ghost blocks (when the game lags).
Setup: Two Repeaters on standard setting creating a clock, sticky piston with a slimeblock on it, block to duplicate (in my screenshots the diamond block) on one side of the slime block. The diamond block gets visually (client-side) duplicated while the piston extends (a copy stays at the previous position). When the piston retracts, the ghost block gets pulled back so that a complete row grows. When you put an additional block at the front of the slime block, both rows also grow in the direction the piston pushes.
This happens whenever the game lags a bit (for me it was ~20fps). F3+A doesn't work, because it's useless, like I said in MC-96142.